The invention relates to a steerable crawler track for an agricultural utility vehicle such as, in particular, a tractor or a self-propelled harvesting machine.
Agricultural utility vehicles, in particular tractors or self-propelled harvesting machines, for example, combine harvesters or forage harvesters, are being equipped with crawler tracks to an increasing extent. The bearing forces of the vehicle can be thereby distributed over a larger ground contact area, which results in a reduced load on the ground over which the vehicle travels.
Due to the large ground contact area of a crawler track and the associated favorable traction conditions, it is advantageous to also transfer drive forces via the crawler track. For this reason, the track roller units associated with crawler tracks of agricultural utility vehicles known from the prior art are usually driven. The drive train of such a crawler track can include a differential, the transmission input of which can be driven via an engine and the two transmission outputs of which drive a track roller unit on the left side and a track roller unit on the right side.
A crawler track known from the subsequently published German patent application 10 2014 104 549.1 is also designed to be steerable. In order to implement the steering function, the transmission outputs of the differential, each of which drives one of the track roller units, are coupled to one another via a superimposed hydrostatic transmission. The ratio of the speeds of the transmission outputs—that is, the ratio of the speeds of the track roller units—can be influenced by changing the transmission ratio of the hydrostatic transmission by changing the intake volume and pump capacity of one or both of the hydrostatic units, thereby creating differential steering for the crawler track in a relatively simple way.
As compared to steerable crawler tracks, the steering function of which is implemented simply by individually braking the track roller unit located on the inside of the curve, the hydraulic superimposed steering system described in the German patent application 10 2014 104 549.1 has the advantage that the steering does not cause brake wear or the associated power losses. In addition, the hydraulic control makes it possible to achieve particularly sensitive steerability.
For heavy agricultural utility vehicles comprising such a steerable crawler track, it has been shown, however, that the hydrostatic units of the hydraulic transmission must be designed to be relatively large in order to generate sufficient torque even at low speeds. The hydrostatic units must also be capable of rotating relatively rapidly, however, so that these vehicles can be driven at relatively high speeds, for example on the road. These two requirements—high torque and high speed—largely cancel each other out, however. In any case, a hydrostatic unit that would satisfy the requirements to the practically required extent would not have acceptable structural dimensions.